This invention relates to the acquisition of seismic data from subterranean formations, and in particular relates to the mapping of shots and receivers for efficient collection of data, for imaging dome-like geological formations.
The conventional pattern of shots and receiver stations for land-based 3D seismic imaging is orthogonal. In an orthogonal brick pattern, several receiver lines are laid out in at spaced intervals generally parallel to one another, and shot lines are laid out in a pattern of lines generally at right angles to the receiver lines. Receiver stations are generally evenly spaced along the receiver lines, and shot stations are generally evenly spaced along the shot lines. An alternate pattern is known as a zig zag, where shot lines cross the receiver lines at angles other than right angles.
When the orthogonal pattern technique is used to plan a survey over a target that includes steeply dipping geological formations, such as salt domes, a significant percentage of the data from the orthogonally patterned survey must be discarded or ignored, including data received by sensors on the opposite side of the dome from where the shots were fired, and data representing signal paths having offsets too long to be meaningful.
One approach to solving the problem of the unsuitability of an orthogonal pattern for imaging salt domes involves mapping receiver lines radially from a point above the center of the dome, and mapping shot or seismic source lines on those same radial lines. To provide more density of shots further away from the center of the dome, over the steeply dipping areas of the target, which are of most interest, the radial method adds more radially-oriented shot lines, beginning some distance away from the center of the dome.
This radial approach has several drawbacks that make it economically impractical. First, there are too many shots and receiver stations located over the shallowest section of the dome--an area of little prospecting interest. Second, the radial approach yields a poor distribution of offsets, i.e. a narrow range of distances between any given source point and an active receiver. This poor distribution of offsets leads to a poor distribution of midpoints, if normal orthogonal bins are used. For this reason, some have tried using unequal bin sizes to provide more midpoints within a bin. The unequal bin sizes require complex mathematics to map bins for migrating data for imaging. Even with this added mathematical complexity, a much larger number of shots than those that would be required for coverage of a particular target using the orthogonal pattern are required, and the fold still does not compare favorably with orthogonal patterns. In addition, changing shot locations from the positions called for in the pattern, or eliminating them, to accommodate surface topography, leads to more unusable data than even the orthogonal pattern.
Another drawback of combining radial shot lines with radial receiver lines is that it results in narrow range of azimuths, that is, a narrow range of directions from a source point to any active receiver. In general, wider ranges of azimuths provide superior image resolution.
What is needed is a pattern that maximizes useable data, and provides a high fold and a wide range of azimuths at a cost comparable to orthogonal survey patterns.